[Core][Doc] Add Description for Named Placement Group to Require a Namespace (#51285)

This PR adds the description for named placement group to require a
namespace. The corresponding code example is updated as well.

Closes #51149

---------

Signed-off-by: Mengjin Yan <mengjinyan3@gmail.com>
Co-authored-by: Dhyey Shah <dhyey2019@gmail.com>

Author: MengjinYan

doc/source/ray-core/doc_code/placement_group_example.py

@@ -129,11 +129,13 @@ def ready(self):
 
 # __get_pg_start__
 # first_driver.py
-# Create a placement group with a global name.
-pg = placement_group([{"CPU": 1}], name="global_name")
+# Create a placement group with a unique name within a namespace.
+# Start Ray or connect to a Ray cluster using: ray.init(namespace="pg_namespace")
+pg = placement_group([{"CPU": 1}], name="pg_name")
 ray.get(pg.ready())
 
 # second_driver.py
-# Retrieve a placement group with a global name.
-pg = ray.util.get_placement_group("global_name")
+# Retrieve a placement group with a unique name within a namespace.
+# Start Ray or connect to a Ray cluster using: ray.init(namespace="pg_namespace")
+pg = ray.util.get_placement_group("pg_name")
 # __get_pg_end__

doc/source/ray-core/scheduling/placement-group.rst

@@ -585,12 +585,19 @@ child tasks and actors to the same placement group, specify ``PlacementGroupSche
 [Advanced] Named Placement Group
 --------------------------------
 
-A placement group can be given a globally unique name.
-This allows you to retrieve the placement group from any job in the Ray cluster.
-This can be useful if you cannot directly pass the placement group handle to
+Within a :ref:`namespace <namespaces-guide>`, you can *name* a placement group.
+You can use the name of a placement group to retrieve the placement group from any job 
+in the Ray cluster, as long as the job is within the same namespace.
+This is useful if you can't directly pass the placement group handle to
 the actor or task that needs it, or if you are trying to
-access a placement group launched by another driver.
-Note that the placement group is still destroyed if its lifetime isn't `detached`.
+access a placement group launched by another driver. 
+
+The placement group is destroyed when the original creation job completes if its 
+lifetime isn't `detached`. You can avoid this by using a :ref:`detached placement group <placement-group-detached>`
+
+Note that this feature requires that you specify a 
+:ref:`namespace <namespaces-guide>` associated with it, or else you can't retrieve the 
+placement group across jobs.
 
 .. tab-set::